From the first operation to repair a heart in 1891 until the early 1950's, heart surgeons were limited by the problem of trying to work on the heart while it was still beating. The heart's constant motion, and the presence of blood that obscured the surgeon's view, made repairing heart defects a surgical challenge. Surgeons had to work quickly and there was always a danger of disrupting blood circulation to vital organs. The solution to this problem came in the late 1950's with the development of the first oxygenator.
The veins return deoxygenated blood to the heart's right atrium. From the right atrium, blood is pumped to the right ventricle, then through the pulmonary artery to the lungs. The lung oxygenates the blood while removing carbon dioxide as it passes through the lung's alveolar capillary network. Oxygenated blood is then returned to the left atrium by way of the pulmonary veins. Blood is then pumped through the mitral valve into the left ventricle and pumped back into the body's circulatory system. Cells are replenished with oxygen and carbon dioxide is taken up by the blood as the blood passes through the body's capillary system. After this gaseous exchange is accomplished, the blood is returned to the heart and the cycle is repeated.
During cardiopulmonary by-pass (CPB) surgery, for example, venous blood is taken from the patient's circulation by means of a cannula placed in the vena cava. The blood "by-passes" the heart and lungs and enters what is referred to as the "extracorporeal circuit" or literally a circuit "outside the body." Oxygenation of the patient's blood takes place in an oxygenator much in the same way as it does in the natural process. After the blood is oxygenated and temperature regulated, it is returned to the patient's arterial circulation through a cannula so that the patient's body may utilize the oxygenated blood.
Early blood oxygenators were called "bubblers" because they bubbled air up through a column of blood, diffusing oxygen into the blood and carbon dioxide out. The problem was that this bubbling action created foam which not only was damaging to the blood, but also rendered this portion useless to the patient.
A defoaming sponge was employed to break down the bubbles of foam back into usable liquid blood. However, the raw sponge by itself was ineffective without a surface agent to lower the surface tension of the blood foam. Silicone Antifoam "A" by Dow Corning was adopted throughout the industry as the defoaming agent of choice. It is comprised of silicone oil and approximately 4.5% silica particles (approximately 5 micrometer (i.e., micron) in diameter). Another development was the use of blood reservoirs to process blood suctioned (scavenged) from the patient during surgery. Antifoam A was used to coat defoaming sponges used in blood reservoirs here as well.
The silicones used in typical silicone antifoams are hydrophobic. Therefore, these compounds are typically dissolved in organic solvents in order to prepare an antifoam solution, which can be applied to a surface to reduce foaming of liquids contacting the surface. The primary solvents that have been used heretofore are chlorofluorocarbons (CFCs) because they are nonflammable and evaporate quickly. Unfortunately, because of recent concerns that CFCs affect the earth's protective ozone layer, the production and use of CFCs will cease in the near future. Thus, there is a need in the art for alternative defoaming compositions having fewer if any detrimental side effects to the individual and the environment.